For example, in an in-vehicle network installed in a vehicle, it is known that the vehicle behaves differently from an intention of a driver, by an unauthorized transmission of a message. As a countermeasure against such an attack to the network, a technology of detecting an unauthorized transmission of a message has been developed. For example, by focusing on periodic transmission of messages, a technology that performs a filtering process on a predetermined transmission period, a technology that detects period abnormalities and that discards a message that is transmitted in an unauthorized manner, and the like have been developed.
However, the main aim of the conventional technology that detects an unauthorized transmission by focusing on the periodic transmission of messages is to determine the legitimacy of a message upon receiving the message. Consequently, when an attempt is made to accurately determine the legitimacy of a message while taking into account the periodic variation and the like, the waiting time is increased and the real-time property is lost. On the other hand, when the real-time property is emphasized, the determination accuracy is lowered, and an unauthorized transmission is apt to be overlooked and misdetection tends to occur. Hence, there is a demand for a new technology capable of accurately determining the presence of an unauthorized transmission of a message without losing the real-time property.